It is often necessary in mining, oil production, engineering and construction to produce holes in rock and other earth formations. Many types of rock drilling equipment use air or some other pneumatic gas which exhausts at the drill bit to bring the earth cuttings to the surface. Rotary conical roller rock bits, blade bits and downhole drilling equipment are examples of drilling equipment types which use exhausting pneumatic gas to remove earth cuttings.
Downhole equipment uses a drill and power unit therefor which progresses into the hole as the rock is removed. Most downhole rock drilling equipment is pneumatically powered using air as the pneumatic gas. Downhole drilling equipment has previously been developed for drilling relatively large diameter holes using drilling pipe having a diameter substantially smaller than the hole. The term large diameter holes is intended to mean holes with diameters of approximately 17 to 36 inches or larger. The small diameter drilling pipe used to drill large holes is commonly about 51/2 inches in diameter although other diameters are used and are considered small if the ratio between the hole diameter and pipe diameter is greater than 2 to 1.
Drilling of these large diameter holes requires large amounts of compressed gas to remove the drill or earth cuttings from the base of the hole. The large amount of gas flow is needed because of the large annular cross-sectional area between the small diameter drilling pipe and the large diameter hole wall. The compressed gas must flow through this large area at a high velocity to entrain and remove the cuttings. A high velocity flow over such a large area requires large amounts of compressed air or other gas.
One prior art approach to reduce the required amount of compressed air involves using a double-walled drilling pipe along the entire drilling string. Such continuous double-walled drilling pipe has a relatively large outside diameter so that the annular cross-sectional flow area is reduced. Since the flow area is reduced, the amount of compressed gas needed to remove the earth cuttings is also reduced.
Large diameter double-walled drilling pipe has been found satisfactory for drilling relatively shallow holes but is unsatisfactory for drilling deep holes. The term deep holes refers to holes in the range of 200 to 1,000 feet or greater. A deep hole drill string made of double-walled pipe requires very large and costly equipment to lift and handle the drill string if it is possible to do so at all. The heavy weight of the drill string also makes it very difficult to accurately control the load placed upon the drilling tool. Excessive loading of the drilling tool causes damage to the tool and premature failure.
Because of these problems it has been difficult and often impossible to drill deep large diameter holes with prior art equipment and drilling methods. When double-walled pipe could not be used because of its weight, it was sometimes possible to use small drilling pipe with large amounts of compressed air to expel the cuttings from the hole. Using such large amounts of compressed air is costly because of the large capital investment in compressor equipment. Capital costs alone are currently about $77 per cubic foot of compressed air capacity. If 51/2 inch drilling pipe is used in a 36 inch hole, the required airflow necessary to produce sufficient velocity is about 13,900 cubic feet per minute. Just the capital investment for providing such large amounts of compressed air is very substantial and is a determinative factor in preventing many small independent drillers from entering the market for drilling large diameter deep holes. A reduction in the required compressed air capacity also reduces the fuel and maintenance costs. Thus it can be seen that there is a great need for reducing the amount of compressed air necessary to drill earth holes, particularly deep, large diameter holes. It can also be understood that greatly reducing the capital equipment cost associated with drilling earth holes will enable a large number of smaller drilling companies to vigorously compete in the market, thereby further reducing drilling costs.